The present invention relates to a compensation device for aiming errors caused by the malfunctioning of phase-shifters of electronic scanning antennas or by the malfunctioning of coefficients of antennas with beam-shaping by computation.
It can be applied notably to electronic scanning antennas when one or more electronically controlled phase-shifters used in the antenna to deflect its beam are malfunctioning, these malfunctions causing a deterioration of the precision of aim in free space of the beam.
In an electronic scanning antenna, the aiming of a beam at a given instant towards a given direction of space is done by acting on the radiation phase of the radiating sources called elementary sources forming the antenna. In order that the changes in the aiming direction may be swift, the modification of the phase of the elementary sources is obtained by the insertion of the electronically controlled phase-shifters series-connected between a microwave power distributor and the elementary sources. A phase-shifter may serve several elementary sources, but the most usually adopted approach is to provide one phase-shifter for each elementary source.
The electronic control of the phase-shifters is done in such a way that the radiated energy gets focused at a great distance in a desired direction. This is done by positioning the different phase-shifters in a certain phase state that is determined a way known to those skilled in the art. Chapter 7 the second edition of Merril I. Skolnick, "Radar Handbook", Mac Grawhill, gives a extensive description of the techniques used and their applications to radar.
It has been shown and verified in practice that, provided that there is a sufficient number of elementary sources, it is not necessary to have a large number of phase states on the phase-shifters in order to obtain efficient performance. In practice, the phase-shifters are therefore controlled by digital data elements in the form of messages giving the phase to be displayed on N bits, which corresponds to 2.sup.N phase positions theoretically spaced out every 360.degree./2.sup.N. The spacing thus corresponds to 45.degree. in the example where N =3 or 22.5.degree. in an example where N=4. Depending on the technology used for the phase-shifter, it is either economically desirable, notably in the case of diode-operated phase-shifters, or unimportant, notably in the case of ferrite phase-shifters, to reduce the number N of bits to the maximum extent, in practice, it is possible to limit the operation to N=1 to 4.
The malfunctions that affect the phase-shifters and the drop in performance resulting therefrom constitute a phenomenon that is accepted rather than resisted. Additional safety margins over what would be necessary for maintaining performance characteristics with all the phase-shifters in operating condition make it possible to cope with this problem. These safety margins are such that, with a given number of malfunctioning phase-shifters, whatever the distribution of these phase-shifters in the antenna, the requisite performance characteristics are always met.
The known methods used to compensate for the effect of malfunctions in phase-shifters therefore consists notably in designing the antenna so that it has performance characteristics which, when there is no malfunctioning in the phase-shifters, are far higher than necessary so that, when there is malfunctioning in the phase-shifters, the requisite performance characteristics are always obtained. The phase-shifters are monitored in order to ascertain, either constantly or at short intervals, that they are truly in working order. The number of suspect phase-shifters is updated and constantly monitored so as to warn the operator or the maintenance services when their number approaches or reaches the maximum number that the system can bear without falling below the requisite performance level. A maintenance operation is needed to replace the suspect phase-shifters.
In the particular case of the MLS system or microwave landing system relating to terminal guidance for landing in airports, using electronic scanning antennas and for which the aiming precision is a priority characteristic, a U.S. Pat. No. 4,041,501 describes a particular embodiment of an electronic scanning MLS antenna and another U.S. Pat. No. 4,359,740 describes the means to cancel the aiming error caused in such an antenna by the malfunctioning of a phase-shifter. The invention described in the latter patent can be applied to diode-operated phase-shifters using, as a 0.degree.-180.degree. cell, a 3 dB coupler connected to two switching diodes controlled independently of each other. As soon as a phase-shifter malfunction is detected, the two diodes of the 0.degree.-180.degree. cell are switched into two distinct states. One of them is then on and therefore in a state of low impedance which may be capacitive. The result is that the microwave signal that goes through the phase-shifter is then cut off or rather greatly attenuated. Consequently, it no longer plays a part in the radiation of the antenna and the aiming error of the beam in free space, resulting from the malfunctioning of the phase-shifter in question, is cancelled. This results directly from the fact that by creating an amplitude gap at the corresponding radiating element, which inhibits the effect of the phase errors, the antenna pattern is deformed symmetrically and the direction of maximum radiation remains unchanged.
These methods have several drawbacks. The methods make it necessary to provide for a margin in the required performance characteristics necessitating a costly design. The associated extra cost is directly related to the desired performance level and to the number of malfunctions that the system can accept. This excess cost may be considerable if the performance level required is high or if the number of malfunctions to be tolerated is great.
The method described in the U.S. Pat. No. 4,359,740 provides a solution only when the phase-shifter is a diode-based phase-shifter and uses a 0.degree.-180.degree. cell comprising two diodes working by reflection of the high frequency signal. Furthermore, it is ineffective when the high frequency signal cut-off device is itself out of order.
The aim of the invention is to overcome the above-mentioned drawbacks, notably by enabling the cancellation of the aiming error in free space due to one or more malfunctions of phase-shifters without its being necessary to complicate these phase-shifters.